Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The Long-Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. For efficient performance, it is central to obtain information about the channel. This information is commonly referred to as channel state information (CSI).
In LTE Release-10, a new reference symbol sequence was introduced for the intent to estimate channel state information, the CSI Reference Symbol (CSI-RS). The CSI-RS provides several advantages over basing the CSI feedback on the Cell-specific Reference Signal (CRS) which were used, for that purpose, in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density, i.e. the overhead of the CSI-RS is substantially less. Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements, e.g., which CSI-RS resource to measure on can be configured in a User Equipment (UE) specific manner. By measuring on a CSI-RS a UE can estimate the effective channel the CSI-RS is traversing including the radio propagation channel and antenna gains.
Moreover, in LTE Rel. 13, a new type of CSI-RS transmitting scheme was introduced, so called beamformed, or precoded, CSI-RS. These CSI-RSs are intended to be UE-specific instead of cell-specific, so that each UE is assigned a dedicated CSI-RS resource. Such beamformed CSI-RS typically contain much fewer ports than non-precoded CSI-RS and typically spans a narrower main beam, as they are typically only intended to cover the UE of interest and not the entire cell coverage area. An alternative way of utilizing precoded, or beamformed, CSI-RS is to transmit a sequence of beamformed CSI-RSs in order to cover the entire cell coverage area, so called beam sweeping. However, a drawback by using a large number of CSI-RS antenna ports is that the overhead becomes larger and there will hence be less resources available for transmitting the actual data. One possible remedy for this is to operate in hybrid CSI-RS reporting mode, where non-precoded CSI-RS, corresponding to a high number of ports, is transmitted rather infrequently in a UE nonspecific manner; it is instead transmitted in a cell specific manner.
Based on the corresponding CSI-RS reports the eNodeB will be able to decide on a set of channel subspaces where different UEs are located. Since the channels will be correlated in time the eNodeB can further utilize the decided channel subspaces by assuming that the UEs will be located within these also in the following subframes. Hence, it may be sufficient for the eNodeB to transmit UE specific beamformed CSI-RS, corresponding to a low number of ports, within the detected subspaces in the following subframes. If the subspaces are small enough and/or the number of UEs are low enough this will result in a reduced overhead compared to transmitting only non-precoded CSI-RS. Hence, the eNodeB will efficiently be able to obtain high quality CSI with an, on average, lower overhead compared to the case where the eNodeB transmits only non-precoded CSI-RS.
When operating in hybrid CSI mode the eNodeB only sees a subspace of the channel space to a UE when obtaining CSI in the second phase due to the choice of channel subspace which was based on the first phase. However, due to the propagation environment, as well as the movement of the UE, it is possible that the CSI obtained in the first phase is outdated and hence the used subspace is no longer beneficial for transmission to the given UE.
When for instance CSI-RS, or a beam sweeping procedure, is used in the first phase of hybrid CSI mode the reference signals are transmitted in a cell-specific manner. However, in a cell containing many UEs these UEs may experience very different propagation environments. Therefore, some UEs may benefit from more frequent cell-specific transmission of reference signals than others. Hence, when deciding on the transmission periodicity for the reference signals in the first phase the requirements for different UEs may be quite different. One may therefore need to decide on a transmission periodicity which constitutes a tradeoff between the requirements of the different UEs in the cell.